Solid-liquid matrices from municipal, industrial and other processes are currently dewatered with a room-temperature belt, filter or screw press. These pieces of equipment employ high-pressure processes during which the water is separated from the solid-liquid matrices.
An energy efficient means to dewater a solid-liquid matrix is through the use of impulse drying. "Impulse drying" occurs when a wet paper web passes through the press nip of a pair of rolls, one of which has been heated to a high temperature. A steam layer adjacent to the heated surface grows and displaces water from the wet sheet of paper in a more efficient manner than conventional evaporative drying.
Impulse drying is described in U.S. Pat. No. 4,324,613. In the method described in this patent, the surface of one of the rolls is heated to a high temperature by an external heat source immediately prior to passing the wet paper web between the heated roll and the other roll. This patent describes the use of solid rolls having at least a surface layer having high thermal conductivity and high thermal diffusivity, such as copper or cast iron, for use as the heated roll.
U.S. Pat. No. 4,324,613 discloses that, in normal cases, a major part of the drying must take place in the press nip, and final drying takes place after the nip. The conductivity of the material of which the heating roll is made must be high so as not to dry at roll surface temperatures higher than necessary. A high conductivity is stated to mean that the heat can be conducted to a greater depth in the roll, and even extracted from a greater depth, which in itself means that a lower roll temperature can be used. U.S. Pat. No. 4,324,613 discloses that the choice of material is limited by the risk of thermal fatigue and, in this respect, at least the surface layer of the roll should be made of a material for which the quantity: ##EQU1##
has a high value desirably at least 0.6.times.10.sup.6, where .sigma..mu. is the fatigue strength, v is Poisson's ratio, .rho. is the density, c is the specific thermal capacity, .lambda. is the thermal conductivity, E is the modulus of elasticity, and a is the coefficient of thermal expansion for the material. Copper alloys are stated to have the highest values, approximately 13.times.10.sup.6. However, they are stated to have rather poor resistance to wear and to not be suitable for doctoring. Other stated suitable materials are duralumin (0.7.times.10.sup.6), cast iron (0.67.times.10.sup.6 -0.85.times.10.sup.6), steel (0.8.times.10.sup.6) and nickel (approximately 0.8.times.10.sup.6 -0.9.times.10.sup.6).
In addition to the impact on energy consumption, impulse drying also has an effect on paper sheet structure and properties. Surface fiber conformability and interfiber bonding are enhanced by transient contact with the hot surface of the roll. As the impulse drying process is usually terminated before the sheet is completely dried, internal flash evaporation results in a distinctive density profile through the sheet that is characterized by dense outer layers and a bulky midlayer. For many paper grades, this translates into improved physical properties. The persistent problem with the use of impulse drying, however, is that flash evaporation can result in delamination of the paper sheet. This is particularly a problem with heavy weight grades of paper. This has been a major constraint as to the commercialization of impulse drying.
U.S. Pat. No. 2,209,759 discloses a press roll assembly having a hard, porous surface roller adapted to receive water pressed from a wet web of paper for conveyance of the water away from the web of paper, and having a second roller. During the conveyance of the water away from the wet web of paper, some of the water is thrown from the roller by centrifugal force, and remaining portions of the water are sucked or blown out of the roller at points spaced from the web of paper by a mechanical suction device cooperating with the outer face of the roller. Column 2, Lines 25.gtoreq.39, on Page 3 of this patent discloses the direction of a flame against the porous surface of the first roller after the removal of water from the web of paper to heat the surface of the roller and continuously supply dewatered and heat-treated pores to the nip of the press roll assembly.
U.S. Pat. No. 2,679,572 discloses a roll having a resilient heated surface for use in drying operations. The heating element which is pressed in the roll is in the form of a layer of electro-conductive plastic composition surrounding an insulating layer, and having sufficient resistance to provide the desired heating action when a difference in electrical potential is maintained across the layer. In order to supply electrical energy or potential to the conductive layer, conductor rings of brass or copper are embedded in the conductive layer. Contact points present in the roll are connected to a suitable source of electrical potential so that a difference of potential is maintained across the conductive layer as a shaft rotates. The resistance of the conductive layer causes heat to be generated uniformly thereover, by which the surface of the roll is heated.
U.S. Pat. No. 4,424,613 described a method and a machine for brushing the pile of a pile fabric, such as a knit fabric, and for removing the wrinkles in a moving web of the material. The wrinkles are removed from the fabric by a wrinkle remover with the application of heat by an infrared heater, and then the fabric is brushed by one or more rotating brushes. The wrinkle remover consists of a pair of rectangular spreader boxes, each of which is connected to a suitable vacuum source through conduit. The vacuum conduit sucks air through an opening to pull the fabric down and maintain it in contact with the bristles of the brushes. As the fabric is being supplied over the spreader boxes, the brushes cam the fabric outward to remove the wrinkles therein as the suction pressure from the vacuum conduit pulls the fabric downward.
U.S. Pat. No. 4,874,469 discloses an apparatus and method in which a formed web is subjected for an extended period of time to increased pressure and temperature, such that fluid within the web is removed therefrom. The apparatus includes a press member (or backing roll), such that when the web passes through the pressing section of the apparatus, fluid is removed from the web, and a heating means which is adjacent to the press member, and which transfers heat to the web. When the web passes through the press section, the web is subjected for an extended period of time to increased pressure and temperature. Water vapor resulting from this high pressure and temperature which is generated in the pressing section of the apparatus during passage of the web therethrough is stated to force the fluid in the liquid phase away from the web. The press member defines a pressing surface which is porous, for inhibiting delamination of the web.
U.S. Pat. No. 4,888,095 discloses a method for extracting water from a wet paper web in a paper making machine using a ceramic foam component which has: (1) a supporting structure; and (2) a water permeable member mounted on the supporting structure which is adapted to support a paper web. The paper web is supported on a moving porous belt, and passes over the water permeable member. When a pressure differential is applied to the wet paper web as it travels over the water permeable member, moisture is extracted from the wet paper web and drains through the water permeable member.
U.S. Pat. No. 5,327,661 and U.S. Pat. No. 5,272,821 disclose a method and apparatus (an electrohydraulic press) for drying a wet web of paper utilizing impulse drying techniques to provide a paper product having a predetermined pattern of delaminated fibers. The wet paper is dried as it passes through the press nip when it is transported through a pair of rolls wherein at least one of the rolls has been heated to an elevated temperature (to a temperature of from about 200.degree. C. to about 500.degree. C.). The heated roll is provided with a planar surface having a predetermined pattern formed on the surface of a material having a low K value of less than about 3000 w.multidot.s/mc, and having a relatively low porosity. The material forming the predetermined pattern of the roll surface is preferably selected from ceramics, polymers, glass, inorganic plastics, composite materials and cermets. The remainder of the roll surface has a high K value of greater than about 3000. The material forming the remainder of the roll surface is preferably selected from steel, molybdenum, nickel and duralumin. The two rolls are urged together to provide a compressive force on the wet paper web as it is transported through the rolls. This method is stated to be useful for the impulse drying of paper webs having an initial moisture level of from about 50% to about 70%. The moisture level of the paper web after being subjected to this impulse drying technique is stated to be in the range of from about 40% to about 60%.
U.S. Pat. No. 5,353,521 and U.S. Pat. No. 5,101,574 disclose a method and apparatus for drying a wet web of paper utilizing impulse drying techniques. The wet paper web is transported through a pair of rolls wherein at least one of the rolls has been heated to an elevated temperature (a temperature of from about 200.degree. C. to about 400.degree. C.) for a residence time of up to about 0.125 seconds. The heated roll is provided with a surface having a low thermal diffusivity of less than about 1.times.10.sup.-6 m.sup.2 /s. The method is stated to be useful for the impulse drying of paper webs having an initial moisture level of from about 50% to about 70%. The moisture level of the paper web after it has been subjected to this impulse drying technique is stated to be in the range of from about 40% to about 60%.